Fusarium wilt of bananas is caused by F. oxysporum f.sp. cubense, a common soil inhabitant. Other formae speciales attack a wide variety of other crops, including cotton, flax, tomatoes, cabbages, peas, sweet potatoes, watermelons and oil palms.;The formae speciales of Fusarium oxysporum each produce three types of asexual spores. The macroconidia (22-36 x 4-5 µm, see Wardlaw, 1961 for measurements) are produced most frequently on branched conidiophores in sporodochia on the surface of infected plant parts or in artificial culture. Macroconidia may also be produced singly in the aerial mycelium, especially in culture. The macroconidia are thin-walled with a definite foot cell and a pointed apical cell. Oval or kidney-shaped microconidia (5-7 x 2.5-3 µm) occur on short microconidiophores in the aerial mycelium and are produced in false heads. Both macroconidia and microconidia may also be formed in the xylem vessel elements of infected host plants, but the microconidia are usually more common. The fungus may be spread by macroconidia, microconidia and mycelium within the plant as well as outside the plant. Illustrations of the conidia have been published (Nelson et al., 1983).;Chlamydospores (9 x 7 µm) are thick-walled asexual spores that are usually produced singly in macroconidia or are intercalary or terminal in the hyphae. The contents are highly refractive. Chlamydospores form in dead host-plant tissue in the final stages of wilt development and also in culture. These spores can survive for an extended time in plant debris in soil.;Mutation in culture is a major problem for those working with vascular wilt isolates of F. oxysporum. The sporodochial type often mutates to a 'mycelial' type or to a 'pionnotal' type. The former has abundant aerial mycelium, but few macroconidia, whereas the latter produces little or no aerial mycelium, but abundant macroconidia. These cultures may lose virulence and the ability to produce toxins. Mutants occur more frequently if the fungus is grown on a medium that is rich in carbohydrates.
Banana;The various symptoms of Fusarium wilt on banana are described and well illustrated by Ploetz and Pegg (1999).;The first external symptoms of Fusarium wilt on bananas is a faint off-green to pale-yellow streak or patch at the base of the petiole of one of the two oldest leaves. The disease can then progress in different ways. The older leaves can yellow, beginning with patches at the leaf margin. Yellowing progresses from the older to the younger leaves until only the recently unfurled or partially unfurled centre leaf remains erect and green. This process may take from 1 to 3 weeks in cultivar 'Gros Michel'. Often the yellow leaves remain erect for 1-2 weeks or some may collapse at the petiole and hang down the pseudostem. In contrast to this 'yellow syndrome', leaves may remain completely green except for a petiole streak or patch but collapse as a result of buckling of the petiole. The leaves fall, the oldest first, until they hang about the plant like a skirt. Eventually, all leaves on infected plants fall down and dry up. The youngest are the last to fall and often stand unusually erect.;Splitting of the base of the pseudostem is another symptom as is necrosis of the emerging heart leaf. Other symptoms include irregular, pale margins on new leaves and the wrinkling and distortion of the lamina. Internodes may also shorten (Stover, 1962, 1972, Jones, 1994, Moore et al., 1995).;The characteristic internal symptom of Fusarium wilt is vascular discoloration. This varies from one or two strands in the oldest and outermost pseudostem leaf sheaths in the early stages of disease to heavy discoloration throughout the pseudostem and fruit stalk in the later disease stages. Discoloration varies from pale yellow in the early stages to dark red or almost black in later stages. The discoloration is most pronounced in the rhizome in the area of dense vascularization where the stele joins the cortex. When symptoms first appear, a small or large portion of the rhizome may be infected. Eventually, almost the entire differentiated vascular system is invaded. The infection may or may not pass into young budding suckers or mature 'daughter' suckers. Where it does, discoloration of vascular strands may be visible in the excised sucker. Usually, suckers less than 1.5 m tall and ca. 4 months old do not show external symptoms. Where wilt is epidemic and spreading rapidly, suckers are usually infected and seldom grow to produce fruit. Above- and below-ground parts of affected plants eventually rot and die.;Fusarium wilt was reported to occur on banana cultivars of the 'Mutika-Lujugira' (AAA genome) subgroup in East Africa above 1400 m. Internal symptoms were much less extensive than those described above and external symptoms more subtle, comprising thin pseudostems and small fingers. Nevertheless, symptomatic plants were recognized by smallholders and were rogued. These mild symptoms were initially believed to be indicative of an attack on a plant whose defences have been weakened as a result of cooler conditions or other predisposing factors at altitude (Ploetz et al., 1994). Given the importance of this banana group, also referred to locally as ÔEast African highland bananasÕ, to local trade and as a staple food, further investigation was merited. This revealed that the disorder also affected non-indigenous banana types, including Cavendish and Bluggoe (which were not affected by Fusarium wilt) and was related to abnormal soil nutrient levels and farm management practice. Discoloration similar to that caused by F. oxysporum f.sp. cubense was observed in vascular tissues of affected plants. Fusarium pallidoroseum (syn. Fusarium semitectum) was consistently isolated from such tissues but found to be non-pathogenic. F. oxysporum was not recovered (Kangire and Rutherford, 2001, Rutherford, 2006).
F. oxysporum f.sp. cubense is one of around 100 formae speciales (special forms) of F. oxysporum which cause vascular wilts of flowering plants (Gerlach and Nirenberg, 1982). Hosts of the various formae speciales are usually restricted to a limited and related set of taxa. As currently defined, F. oxysporum f.sp. cubense affects the following species in the order Zingiberales: in the family Musaceae, Musa acuminata, M. balbisiana, M. schizocarpa and M. textilis, and in the family Heliconeaceae, Heliconia caribaea, H. chartacea, H. crassa, H. collinsiana, H. latispatha, H. mariae, H. rostrata and H. vellerigera (Stover, 1962, Waite, 1963). Additional hosts include hybrids between M. acuminata and M. balbisiana, and M. acuminata and M. schizocarpa.;F. oxysporum f.sp. cubense may survive as a parasite of non-host weed species. Three species of grass (Paspalum fasciculatum, Panicum purpurascens [ Brachiaria mutica ] and Ixophorus unisetus) and Commelina diffusa have been implicated (Waite and Dunlap, 1953).
B. phoenicis is a highly variable species, but can be readily distinguished in the adult stage from the other members of the genus by having five pairs of dorsolateral hysterosomal setae and two sensory rods on tarsus II. The chaetotaxy of false spider mite is described according to Haramoto (1969).
The larvae and nymphs also have five pairs of dorsolateral hysterosomal setae, but unlike the adults they have only one sensory rod, located posteriodistally on tarsus II. Morphologically, the immature stages of B. phoenicis resemble those of B. obovatus, and like the latter they are subjected to considerable variation in size and shape of some of the dorsal setae. The number and arrangement of the setae on the dorsum of idiosoma of the larva, protonymph and deutonymph conform to those of the adult and to the genus Brevipalpus (Pritchard and Baker, 1951). Twelve pairs of setae are present on the dorsum, three pairs on the propodosoma and nine pairs on the hysterosoma. Of the dorsal setae, dorsolateral hysterosomal setae I and II and dorsocentral hysterosomal seta III are the most variable in size and shape, varying from tiny and serrate to large, broadly lanceolate and serrate, similar to dorsal propodosomal setae II and III. The number of setae on the venter is not constant but increases from four pairs on the larva, five pairs on the protonymph, seven pairs on the deutonymph to eight pairs on the adult. These additions take place in the hysterosomal regions of the body. A pair of medioventral opisthosomal setae is present in the two nymphal and adult stages but not in the larval stage. The medioventral propodosomal setae, which are present in the larval, nymphal and adult stages, and the posterior medioventral metapodosomal setae, which are present only in the deutonymphal and adult stages, are filamentous and smooth. The remaining ventral setae are smooth.
In general, the size of an individual of a stage varies according to the availability of resources (Dosse, 1952). The average size (in µm) of different stages is given below according to Nageshchandra and Channabasavanna (1974b).
Body dimensions (µm):
Egg, L 90, W 59;larva, L 145, W 102;protonymph, L 192, W 115;deutonymph, L 238, W 135;adult male, L 268, W 135;adult female, L 277, W 140.
Length of legs (µm):
Larva, I 61, II 45, III 42;protonymph, I 77, II 61, III 52, IV 56;deutonymph, I 100, II 82, III 75, IV 77;adult male, I 147, II 126, III 112, IV 130;adult female, I 142, II 121, III 114, IV 121. These values reveal that the growth of this mite is rather rapid until it reaches the deutonymphal stage, and then is more or less static. The reason might be that in the adult stage most of the food consumed is utilized for the production of eggs, whereas in the earlier stages it is used for its own growth. These mites live longer than other Tetranychid mites, but are half the size.
A diagnostic Lucid key to 19 species of Brevipalpus is available in Flat Mites of the World.
Although false spider mite is considered to be polyphagous, it is thought to cause serious crop losses on citrus (Muma, 1964;Knorr and Denmark, 1970), tea (Baptist and Ranaweera, 1955;Rao, 1970;Danthanarayana and Ranaweera, 1972;Kalshoven and van der Laan, 1981;Oomen, 1982) and papaya (Haramoto, 1969). Losses on citrus due to this mite can be enormous.
Mites belonging to the family Tenuipalpidae feed in the same way as the Tetranychidae, by continually punching the leaf epidermis with their chelicerae (Jeppson et al., 1975). The sap that oozes out of the wounded leaf cells is mixed with saliva and imbibed into the digestive tract of the mite (Haramoto, 1969). The necrotic spots are visible as a brownish, shaded area on the affected leaves, and the affected leaf can be seen filled with red coloured eggs and white empty moults (Oomen, 1982).
In tea, damage is caused by sucking sap from the stems and leaves, producing a characteristic necrotic brown spot extending along the midribs and borders of the leaves. The whole underside becomes brown, which may lead to defoliation and subsequently reduce the production of green tea leaves (Benjamin, 1968;Oomen, 1982). It is typical for affected bushes to have a thin canopy of maintenance leaves causing increased light penetration into the frame of the bush, which permits the growth of mosses and lichens. This is a traditional indication to planters that the tea bushes are in poor condition, although they are often not aware of the relation with false spider mite infestation.
On papaya plants, this mite usually feeds on the trunk at the level where the bottom whorl of leaves is attached. As intraspecific competition for food and space intensifies, the mites feed upwards on the trunk and outwards onto the leaf petioles and fruits, leaving a large, conspicuous, damaged area behind them. The immediate area around the feeding puncture becomes raised and blister-like, as though caused by a toxic substance. Later the affected tissue dries up, dies and becomes discoloured. As many punctures occur close together, the affected areas coalesce to form a large and continuous area which is callous-like, tannish and scaly and/or scabby. The feeding becomes pronounced when young papaya fruits are attacked, as the affected areas become sunken due to the differential growth of the injured and uninjured tissues. The mites sometimes puncture the latex glands while feeding, causing a copious outflow of a milky white liquid that mars the appearance of the fruits. All stages of the mite in the path of the flow of sticky latex are engulfed and drowned in it. The papaya stem, which normally remains green for a long time, becomes tannish and suberized in appearance, and makes spindly growth when heavily infested by B. phoenicis (Haramoto, 1969).
The damage caused by false spider mite is of a higher magnitude on citrus than any other crop plants, including tea and papaya. On citrus, it can cause Brevipalpus gall and halo scab with phoenicis blotch - a combination of fungus and mite attack (Knorr and Denmark, 1970;Carter, 1973). Plants attacked by the mite produce galls in the nodal region, which eventually the hinder the sprouting of new buds. The gall-like protuberances may be barely visible and woody. They look like axes that have proliferated to resemble a bud-studded cushion. No leaves develop at the axis occupied by these cushions, and when the buds are replaced by the cushions the trees become devoid of leaves and soon die. Gall formation follows the initial loss of leaves;adventitious buds sprout but are successively killed, producing hypertrophies at the bud loci (Knorr et al., 1960;Knorr, 1964;Jeppson et al., 1975). This type of symptom is common among seedlings, which ultimately die. The fungus Elsinoë fawcetti causes scab on sour orange without causing leaf drop;however, when scab lesions are also colonized by B. phoenicis, leaf drop is conspicuous. The combination of fungus and mite attack in the nursery could pose a serious problem. High populations of false spider mite are associated with a diffuse chlorotic spotting in orange trees. When such chlorotic spottings increase, they reduce the area of photosynthesis, ultimately reducing fruit production. As well as causing feeding damage, the mite transmits a viral disease caused by citrus leprosis rhabdovirus (Kitajima et al., 1972;Carter, 1973). A disease of oranges known in Argentina as 'lepra explosiva', originally thought to be caused by a fungus (Marchionatto, 1935) and later by a virus (Marchionatto, 1938;Blanchard, 1939), is now attributed to toxins injected by Brevipalpus obovatus in the process of feeding (Carter, 1952). B. phoenicis has also been collected from orange trees exhibiting these symptoms in Paraguay (Nickel, 1958).
In addition to these symptoms, B. phoenicis can cause pitting and splitting of the skin of orange fruits (Planes, 1954);scarring of tangerine fruits (Nickel, 1958);defoliation and vine dieback of passion fruit (Haramoto, 1969);and splitting of guava fruits (Nageshchandra and Channabasavanna, 1974b). It also transmits coffee ringspot virus disease on coffee (Chagas, 1973).
The first report of a host plant of B. phoenicis was Phoenix sp., a greenhouse palm (Geijskes, 1939). Since then, many different plants have been reported as infested by this species of mite in different parts of the world (Cromroy, 1958;Pritchard and Baker, 1958;Baker and Pritchard, 1960;de Leon, 1961;Rimoando, 1962;Haramoto, 1969;Nageshchandra and Channabasavanna, 1974a). Of these, Pritchard and Baker (1958) listed 63 host plant genera, and Nageshchandra and Channabasavanna (1974a) listed 35 genera in India alone. Jeppson et al. (1975) listed citrus, tea, coffee, peach, papaya, loquat, coconut, apple, pear, guava, olive, fig, walnut and grape as its principal hosts.
A toxin is a poisonous substance produced within living cells or organisms; synthetic toxicants created by artificial processes are thus excluded. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived from the word toxic.
Toxins can be small molecules, peptides, or proteins that are capable of causing disease on contact with or absorption by body tissues interacting with biological macromolecules such as enzymes or cellular receptors. Toxins vary greatly in their toxicity, ranging from usually minor (such as a bee sting) to almost immediately deadly (such as botulinum toxin).
Poisons produced by certain microorganisms, plants or animals that are often proteins.